In contrary to the widely reported single and symmetric peak feature of G' band in Raman spectrum of graphene, we herein report the observation of splitting in G' band in free standing graphene. Our experimental findings provide a direct and strong support for the previous theoretical prediction that the coexistence of the outer and inner processes in the double-resonance Raman scattering would cause the splitting of G' mode. The investigation of the influence of trigonal warping effect on the spectral features of G' subbands further verified the theoretical interpretation established on the anisotropic electronic structure of graphene. (C) 2012 American Institute of Physics. [http://dx.doi.org.libproxy1.nus.edu.sg/10.1063/1.4729407]

By using nonequilibrium molecular dynamics simulations, we demonstrated that thermal conductivity of germanium nanowires can be reduced more than 25% at room temperature by atomistic coating. There is a critical coating thickness beyond which thermal conductivity of the coated nanowire is larger than that of the host nanowire. The diameter-dependent critical coating thickness and minimum thermal conductivity are explored. Moreover, we found that interface roughness can induce further reduction of thermal conductivity in coated nanowires. From the vibrational eigenmode analysis, it is found that coating induces localization for low-frequency phonons, while interface roughness localizes the high-frequency phonons. Our results provide an available approach to tune thermal conductivity of nanowires by atomic layer coating.

We describe light scattering from a graphene sheet having a modulated optical conductivity. We show that such modulation enables the excitation of surface plasmon polaritons by an electromagnetic wave impinging at normal incidence. The resulting surface plasmon polaritons are responsible for a substantial increase of electromagnetic radiation absorption by the graphene sheet. The origin of the modulation can be due either to a periodic strain field or to adatoms (or absorbed molecules) with a modulated adsorption profile.

@article{lu_transforming_2012,
title = {Transforming moire blisters into geometric graphene nano-bubbles},
author = {Lu, Jiong and Castro Neto, A. H. and Loh, Kian Ping},
doi = {10.1038/ncomms1818},
issn = {2041-1723},
year = {2012},
date = {2012-05-01},
journal = {Nat. Commun.},
volume = {3},
pages = {823},
abstract = {Strain engineering has been proposed as an alternative method for manipulating the electronic properties of graphene. However, the bottleneck for strain engineering in graphene has been the ability to control such strain patterns at the nanoscale. Here we show that high level of control can be accomplished by chemically modifying the adherence of graphene on metal. Using scanning tunnelling microscopy, the shape evolution of graphene Moire blisters towards geometrically well-defined graphene bubbles was studied during the controlled, sub-layer oxidation of the ruthenium substrate. Understanding the dynamics of the oxidation process and defects generation on the Ru substrate allows us to control the size, shape and the density of the bubbles and its associated pseudo-magnetism. We also show that a modification of the same procedure can be used to create antidots in graphene by catalytic reaction of the same nanobubbles.},
note = {WOS:000304611400019},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

Strain engineering has been proposed as an alternative method for manipulating the electronic properties of graphene. However, the bottleneck for strain engineering in graphene has been the ability to control such strain patterns at the nanoscale. Here we show that high level of control can be accomplished by chemically modifying the adherence of graphene on metal. Using scanning tunnelling microscopy, the shape evolution of graphene Moire blisters towards geometrically well-defined graphene bubbles was studied during the controlled, sub-layer oxidation of the ruthenium substrate. Understanding the dynamics of the oxidation process and defects generation on the Ru substrate allows us to control the size, shape and the density of the bubbles and its associated pseudo-magnetism. We also show that a modification of the same procedure can be used to create antidots in graphene by catalytic reaction of the same nanobubbles.

The electromagnetic enhancement for surface enhanced Raman spectroscopy (SERS) of graphene is studied by inserting a layer of Al2O3 between epitaxial graphene and Au nanoparticles. Different excitation lasers are utilized to study the relationship between laser wavelength and SERS. The theoretical calculation shows that the extinction spectrum of Au nanoparticles is modulated by the presence of graphene. The experimental results of the relationship between the excitation laser wavelength and the enhancement factor fit well with the calculated results. An exponential relationship is observed between the enhancement factor and the thickness of the spacer layer. (C) 2012 American Institute of Physics. [http://dx.doi.org.libproxy1.nus.edu.sg/10.1063/1.4712054]

Graphene has been hailed as a wonderful material in electronics, and recently, it is the rising star in photonics, as well. The wonderful optical properties of graphene afford multiple functions of signal emitting, transmitting, modulating, and detection to be realized in one material. In this paper, the latest progress in graphene photonics, plasmonics, and broadband optoelectronic devices is reviewed. Particular emphasis is placed on the ability to integrate graphene photonics onto the silicon platform to afford broadband operation In light routing and amplification, which involves components like polarizer, modulator, and photodetector. Other functions like saturable absorber and optical limiter are also reviewed.

Graphene has exceptional optical, mechanical, and electrical properties, making it an emerging material for novel optoelectronics, photonics, and flexible transparent electrode applications. However, the relatively high sheet resistance of graphene is a major constraint for many of these applications. Here we propose a new approach to achieve low sheet resistance in large-scale CVD monolayer graphene using nonvolatile ferroelectric polymer gating. In this hybrid structure, large-scale graphene is heavily doped up to 3 x 10(13) cm(-2) by nonvolatile ferroelectric dipoles, yielding a low sheet resistance of 120 Omega/square at ambient conditions. The graphene-ferroelectric transparent conductors (GFeTCs) exhibit more than 95% transmittance from the visible to the near-infrared range owing to the highly transparent nature of the ferroelectric polymer. Together with its excellent mechanical flexibility, chemical inertness, and the simple fabrication process of ferroelectric polymers, the proposed GFeTCs represent a new route toward large-scale graphene-based transparent electrodes and optoelectronics.

The differences between spin relaxation in graphene and in other materials are discussed. For relaxation by scattering processes, the Elliot-Yafet mechanism, the relation between the spin and the momentum scattering times, acquires a dependence on the carrier density, which is independent of the scattering mechanism and the relation between mobility and carrier concentration. This dependence puts severe restrictions on the origin of the spin relaxation in graphene. The density dependence of the spin relaxation allows us to distinguish between ordinary impurities and defects which modify locally the spin-orbit interaction.

The electromagnetic mode spectrum of single-layer graphene subjected to a quantizing magnetic field is computed taking into account intraband and interband contributions to the magneto-optical conductivity. We find that a sequence of weakly decaying quasi-transverse-electric modes, separated by magnetoplasmon polariton modes, emerge due to the quantizing magnetic field. The characteristics of these modes are tunable by changing the magnetic field or the Fermi energy.

Pyridine-functionalized graphene (reduced graphene oxide) can be used as a building block in the assembly of metal organic framework (MOF). By reacting the pyridine-functionalized graphene with iron-porphyrin, a graphene-metalloporphyrin MOF with enhanced catalytic activity for oxygen reduction reactions (ORB) is synthesized. The structure and electrochemical property of the hybrid MOF are investigated as a function of the weight percentage of the functionalized graphene added to the iron-porphyrin framework. The results show that the addition of pyridine-functionalized graphene changes the crystallization process of iron-porphyrin in the MOF, increases its porosity, and enhances the electrochemical charge transfer rate of iron-porphyrin. The graphene-metalloporphyrin hybrid shows facile 4-electron ORR and can be used as a promising Pt-free cathode in alkaline Direct Methanol Fuel Cell.

Edge nanoscrolls are shown to strongly influence transport properties of suspended graphene in the quantum Hall regime. The relatively long arclength of the scrolls in combination with their compact transverse size results in formation of many nonchiral transport channels in the scrolls. They short circuit the bulk current paths and inhibit the observation of the quantized two-terminal resistance. Unlike competing theoretical proposals, this mechanism of disrupting the Hall quantization in suspended graphene is not caused by ill-chosen placement of the contacts, singular elastic strains, or a small sample size.

Highly sensitive, multicomponent broadband photodetector devices are made from PbSe/ graphene/ TiO2. TiO2 and PbSe nanoparticles act as light harvesting photoactive materials from the UV to IR regions of the electromagnetic spectrum, while the graphene acts as a charge collector for both photogenerated holes and electrons under an applied electric field.

We investigate the electronic properties of ultrathin hexagonal boron nitride (h-BN) crystalline layers with different conducting materials (graphite, graphene, and gold) on either side of the barrier layer. The tunnel current depends exponentially on the number of h-BN atomic layers, down to a monolayer thickness. Conductive atomic force microscopy scans across h-BN terraces of different thickness reveal a high level of uniformity in the tunnel current. Our results demonstrate that atomically thin h-BN acts as a defect-free dielectric with a high breakdown field. It offers great potential for applications in tunnel devices and in field-effect transistors with a high carrier density in the conducting channel.

@article{ferreira_graphene-based_2012,
title = {Graphene-based photodetector with two cavities},
author = {Ferreira, Aires and Peres, N. M. R. and Ribeiro, R. M. and Stauber, T.},
doi = {10.1103/PhysRevB.85.115438},
issn = {1098-0121},
year = {2012},
date = {2012-03-01},
journal = {Phys. Rev. B},
volume = {85},
number = {11},
pages = {115438},
abstract = {We present an efficient graphene-based photodetector with two Fabri-Perot cavities. It is shown that the absorption can reach almost 100% around a given frequency, which is determined by the two-cavity lengths. It is also shown that hysteresis in the absorbance is possible, with the transmittance amplitude of the mirrors working as an external driving field. The role of nonlinear contributions to the optical susceptibility of graphene is discussed.},
note = {WOS:000301916100008},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

We present an efficient graphene-based photodetector with two Fabri-Perot cavities. It is shown that the absorption can reach almost 100% around a given frequency, which is determined by the two-cavity lengths. It is also shown that hysteresis in the absorbance is possible, with the transmittance amplitude of the mirrors working as an external driving field. The role of nonlinear contributions to the optical susceptibility of graphene is discussed.

The self-assembly of nonplanar chloroaluminum phthalocyanine (ClAlPc) molecules as well-ordered single-molecule dipole arrays on the silicon carbide (SiC) nanomesh substrate was investigated using low temperature scanning tunneling microscopy. ClAlPc exclusively adsorbs in the center of the SiC nanomesh holes with its inherent dipole (from Cl to Al) pointing toward the substrate. The dipole can be inverted by a positively biased tip with a threshold tip voltage of 3.3 V. We deduce that the interaction between the intrinsic dipole of ClAlPc and the periodic out-of-plane component of the surface dipole on the SiC nanomesh plays a significant role in the dipole array formation.

@article{kitt_lattice-corrected_2012,
title = {Lattice-corrected strain-induced vector potentials in graphene},
author = {Kitt, Alexander L. and Pereira, Vitor M. and Swan, Anna K. and Goldberg, Bennett B.},
doi = {10.1103/PhysRevB.85.115432},
issn = {1098-0121},
year = {2012},
date = {2012-03-01},
journal = {Phys. Rev. B},
volume = {85},
number = {11},
pages = {115432},
abstract = {The electronic implications of strain in graphene can be captured at low energies by means of pseudovector potentials which can give rise to pseudomagnetic fields. These strain-induced vector potentials arise from the local perturbation to the electronic hopping amplitudes in a tight-binding framework. Here we complete the standard description of the strain-induced vector potential, which accounts only for the hopping perturbation, with the explicit inclusion of the lattice deformations or, equivalently, the deformation of the Brillouin zone. These corrections are linear in strain and are different at each of the strained, inequivalent Dirac points, and hence are equally necessary to identify the precise magnitude of the vector potential. This effect can be relevant in scenarios of inhomogeneous strain profiles, where electronic motion depends on the amount of overlap among the local Fermi surfaces. In particular, it affects the pseudomagnetic field distribution induced by inhomogeneous strain configurations, and can lead to new opportunities in tailoring the optimal strain fields for certain desired functionalities.},
note = {WOS:000301837400006},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

The electronic implications of strain in graphene can be captured at low energies by means of pseudovector potentials which can give rise to pseudomagnetic fields. These strain-induced vector potentials arise from the local perturbation to the electronic hopping amplitudes in a tight-binding framework. Here we complete the standard description of the strain-induced vector potential, which accounts only for the hopping perturbation, with the explicit inclusion of the lattice deformations or, equivalently, the deformation of the Brillouin zone. These corrections are linear in strain and are different at each of the strained, inequivalent Dirac points, and hence are equally necessary to identify the precise magnitude of the vector potential. This effect can be relevant in scenarios of inhomogeneous strain profiles, where electronic motion depends on the amount of overlap among the local Fermi surfaces. In particular, it affects the pseudomagnetic field distribution induced by inhomogeneous strain configurations, and can lead to new opportunities in tailoring the optimal strain fields for certain desired functionalities.

@article{mao_silicon_2012,
title = {Silicon layer intercalation of centimeter-scale, epitaxially grown monolayer graphene on Ru(0001)},
author = {Mao, Jinhai and Huang, Li and Pan, Yi and Gao, Min and He, Junfeng and Zhou, Haitao and Guo, Haiming and Tian, Yuan and Zou, Qiang and Zhang, Lizhi and Zhang, Haigang and Wang, Yeliang and Du, Shixuan and Zhou, Xingjiang and Castro Neto, A. H. and Gao, Hong-Jun},
doi = {10.1063/1.3687190},
issn = {0003-6951},
year = {2012},
date = {2012-02-01},
journal = {Appl. Phys. Lett.},
volume = {100},
number = {9},
pages = {093101},
abstract = {We develop a strategy for graphene growth on Ru(0001) followed by silicon-layer intercalation that not only weakens the interaction of graphene with the metal substrate but also retains its superlative properties. This G/Si/Ru architecture, produced by silicon-layer intercalation approach (SIA), was characterized by scanning tunneling microscopy/spectroscopy and angle resolved electron photoemission spectroscopy. These experiments show high structural and electronic qualities of this new composite. The SIA allows for an atomic control of the distance between the graphene and the metal substrate that can be used as a top gate. Our results show potential for the next generation of graphene-based materials with tailored properties. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3687190]},
note = {WOS:000301504800048},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

We develop a strategy for graphene growth on Ru(0001) followed by silicon-layer intercalation that not only weakens the interaction of graphene with the metal substrate but also retains its superlative properties. This G/Si/Ru architecture, produced by silicon-layer intercalation approach (SIA), was characterized by scanning tunneling microscopy/spectroscopy and angle resolved electron photoemission spectroscopy. These experiments show high structural and electronic qualities of this new composite. The SIA allows for an atomic control of the distance between the graphene and the metal substrate that can be used as a top gate. Our results show potential for the next generation of graphene-based materials with tailored properties. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3687190]

An obstacle to the use of graphene as an alternative to silicon electronics has been the absence of an energy gap between its conduction and valence bands, which makes it difficult to achieve low power dissipation in the OFF state. We report a bipolar field-effect transistor that exploits the low density of states in graphene and its one-atomic-layer thickness. Our prototype devices are graphene heterostructures with atomically thin boron nitride or molybdenum disulfide acting as a vertical transport barrier. They exhibit room-temperature switching ratios of approximate to 50 and approximate to 10,000, respectively. Such devices have potential for high-frequency operation and large-scale integration.

The electronic properties of graphene sheets decorated with nanodiamond (ND) particles have been investigated. The chemical fusion of ND to the graphene lattice creates pockets of local defects with robust interfacial bonding. At the ND-bonded regions, the atoms of graphene lattice follow sp(3)-like bonding, and such regions play the role of conduction bottlenecks for the percolating sp(2) graphene network. The low-temperature charge transport reveals an insulating behavior for the disordered system associated with Anderson localization for the charge carriers in graphene. A large negative magnetoresistance is observed in this insulating regime, and its origin is discussed in the context of magnetic correlations of the localized charge carriers with local magnetic domains and extrinsic metal impurities associated with the ND.

The technical breakthrough in synthesizing graphene by chemical vapor deposition methods (CVD) has opened up enormous opportunities for large-scale device applications. To improve the electrical properties of CVD graphene grown on copper (Cu-CVD graphene), recent efforts have focused on increasing the grain size of such polycrystalline graphene films to 100 mu m and larger. While an increase in grain size and, hence, a decrease of grain boundary density is expected to greatly enhance the device performance, here we show that the charge mobility and sheet resistance of Cu-CVD graphene Is already limited within a single grain. We find that the current high-temperature growth and wet transfer methods of CVD graphene result in quasi-periodic nanoripple arrays (NRAs). Electron-flexural phonon scattering in such partially suspended graphene devices Introduces anisotropic charge transport and sets limits to both the highest possible charge mobility and lowest possible sheet resistance values. Our findings provide guidance for further improving the CVD graphene growth and transfer process.

@article{tang_highly_2012,
title = {Highly Wrinkled Cross-Linked Graphene Oxide Membranes for Biological and Charge-Storage Applications},
author = {Tang, Lena A. L. and Lee, Wong Cheng and Shi, Hui and Wong, Ethel Y. L. and Sadovoy, Anton and Gorelik, Sergey and Hobley, Jonathan and Lim, Chwee Teck and Loh, Kian Ping},
doi = {10.1002/smll.201101690},
issn = {1613-6810},
year = {2012},
date = {2012-02-01},
journal = {Small},
volume = {8},
number = {3},
pages = {423--431},
abstract = {Inspired by the amphiphilicity of graphene oxide (GO), the surface of water is used as a template for the assembly of a GO film. Methacrylate-functionalized GO sheets can be cross-linked instantaneously at the waterair interface to form a highly wrinkled membrane spreading over an extended area. The multiple covalent linkages amongst the GO sheets enhances the in-plane stiffness of the film compared to noncovalently bonded GO films. The highly convoluted GO membrane can be used in two applications: the promoting of spontaneous stem-cell differentiation towards bone cell lineage without any chemical inducers, and for supercapacitor electrodes. Due to reduced van der Waals restacking, capacitance values up to 211 F g-1 can be obtained. The scalable and inexpensive nature of this assembly route enables the engineering of membranes for applications in regenerative medicine and energy-storage devices where secondary structures like nanotopography and porosity are important performance enhancers.},
note = {WOS:000299621500013},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

Inspired by the amphiphilicity of graphene oxide (GO), the surface of water is used as a template for the assembly of a GO film. Methacrylate-functionalized GO sheets can be cross-linked instantaneously at the waterair interface to form a highly wrinkled membrane spreading over an extended area. The multiple covalent linkages amongst the GO sheets enhances the in-plane stiffness of the film compared to noncovalently bonded GO films. The highly convoluted GO membrane can be used in two applications: the promoting of spontaneous stem-cell differentiation towards bone cell lineage without any chemical inducers, and for supercapacitor electrodes. Due to reduced van der Waals restacking, capacitance values up to 211 F g-1 can be obtained. The scalable and inexpensive nature of this assembly route enables the engineering of membranes for applications in regenerative medicine and energy-storage devices where secondary structures like nanotopography and porosity are important performance enhancers.

We analyze the results of scanning near-field infrared spectroscopy performed on thin films of a-SiO2 on Si substrate. The measured near-field signal exhibits surface-phonon resonances whose strength has a prominent thickness dependence in the range from 2 to 300 nm. These observations are compared with calculations in which the tip of the near-field infrared spectrometer is modeled either as a point dipole or an elongated spheroid. The latter model accounts for the antenna effect of the tip and gives a better agreement with the experiment. Possible applications of the near-field technique for depth profiling of layered nanostructures are discussed.

CuCo2O4 and CuO center dot Co3O4 compounds were prepared by a one-pot simple molten salt method (MSM) at 280 degrees C to 750 degrees C. Changes in morphology, crystal structure and electrochemical properties of CuCo2O4 as a function of preparation temperatures were investigated using X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Brunauer-Emmett- Teller absorption isotherm. XRD patterns of the sample prepared at 280 degrees C show a crystalline cubic structure with a lattice parameter value of a = 8.131 angstrom and a surface area value of 9.8 m(2) g(-1). The sample prepared at temperatures textgreater510 degrees C shows the presence of CuO center dot Co3O4 phases. Energy storage properties are evaluated using cyclic voltammetry (CV) and galvanostatic cycling studies. CV studies show a main anodic peak at similar to 2.1 V and cathodic peak at similar to 1.2 V. At a current rate of 60 mA g(-1) and in the voltage range of 0.005-3.0 V vs. Li, CuCo2O4 composite prepared at 510 degrees C shows a high and stable capacity of similar to 680 (quenched) and 740 (slow cooling) mAh g(-1) at the end of the 40th cycle.

@article{novoselov_two-dimensional_2012,
title = {Two-dimensional crystals-based heterostructures: materials with tailored properties},
author = {Novoselov, K. S. and Castro Neto, A. H.},
doi = {10.1088/0031-8949/2012/T146/014006},
issn = {0031-8949},
year = {2012},
date = {2012-01-01},
journal = {Phys. Scr.},
volume = {T146},
pages = {014006},
abstract = {Graphene is just one example of a large class of two-dimensional crystals. These crystals can either be extracted from layered three-dimensional materials or grown artificially by several different methods. Furthermore, they present physical properties that are unique because of the low dimensionality and their special crystal structure. They have potential for semiconducting behavior, magnetism, superconductivity, and even more complex many-body phenomena. Two-dimensional crystals can also be assembled in three-dimensional heterostructures that do not exist in nature and have tailored properties, opening an entirely new chapter in condensed matter research.},
note = {WOS:000300504800007},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

Graphene is just one example of a large class of two-dimensional crystals. These crystals can either be extracted from layered three-dimensional materials or grown artificially by several different methods. Furthermore, they present physical properties that are unique because of the low dimensionality and their special crystal structure. They have potential for semiconducting behavior, magnetism, superconductivity, and even more complex many-body phenomena. Two-dimensional crystals can also be assembled in three-dimensional heterostructures that do not exist in nature and have tailored properties, opening an entirely new chapter in condensed matter research.

The graphene Moire superstructure offers a complex landscape of humps and valleys to molecules adsorbing and diffusing on it. Using C-60 molecules as the classic hard sphere analogue, we examine its assembly and layered growth on this corrugated landscape. At the monolayer level, the cohesive interactions of C-60 molecules adsorbing on the Moire lattice freeze the molecular rotation of C-60 trapped in the valley sites, resulting in molecular alignment of all similarly trapped C-60 molecules at room temperature. The hierarchy of adsorption potential well on the Moire lattice causes diffusion-limited dendritic growth of C-60 films, as opposed to isotropic growth observed on a smooth surface like graphite. Due to the strong binding energy of the C-60 film, part of the dentritic C-60 films polymerize at 850 K and act as solid carbon sources for graphene homoepitaxy. Our findings point to the possibility of using periodically corrugated graphene in molecular spintronics due to its ability to trap and align organic molecules at room temperature.

@article{lopes_chiral_2011,
title = {Chiral filtering in graphene with coupled valleys},
author = {Lopes, P. L. e S. and Castro Neto, A. H. and Caldeira, A. O.},
doi = {10.1103/PhysRevB.84.245432},
issn = {1098-0121},
year = {2011},
date = {2011-12-01},
journal = {Phys. Rev. B},
volume = {84},
number = {24},
pages = {245432},
abstract = {We analyze the problem of electronic transmission through different regions of a graphene sheet that are characterized by different types of connections between the Dirac points. These valley symmetry breaking Hamiltonians might arise from electronic self-interaction mediated by the dielectric environment of distinct parts of the substrate on which the graphene sheet is placed. We show that it is possible to have situations in which we can use these regions to select or filter states of one desired chirality.},
note = {WOS:000298116400011},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

We analyze the problem of electronic transmission through different regions of a graphene sheet that are characterized by different types of connections between the Dirac points. These valley symmetry breaking Hamiltonians might arise from electronic self-interaction mediated by the dielectric environment of distinct parts of the substrate on which the graphene sheet is placed. We show that it is possible to have situations in which we can use these regions to select or filter states of one desired chirality.

The electronic properties of graphene are strongly influenced by electrostatic forces arising from long-range charge scatterers and by changes in the local dielectric environment. This makes graphene extremely sensitive to the surface charge density of cells interfacing with it Here, we developed a graphene transistor array integrated with microfluidic flow cytometry for the "flow-catch-release" sensing of malaria-infected red blood cells at the single-cell level. Malaria-infected red blood cells induce highly sensitive capacitively coupled changes in the conductivity of graphene. Together with the characteristic conductance dwell times, specific microscopic information about the disease state can be obtained.

We show that by enclosing graphene in an optical cavity, giant Faraday rotations in the infrared regime are generated and measurable Faraday rotation angles in the visible range become possible. Explicit expressions for the Hall steps of the Faraday rotation angle are given for relevant regimes. In the context of this problem we develop an equation of motion (EOM) method for calculation of the magneto-optical properties of metals and semiconductors. It is shown that properly regularized EOM solutions are fully equivalent to the Kubo formula.

We address the quantum Hall behavior in twisted bilayer graphene transferred from the C face of SiC. The measured Hall conductivity exhibits the same plateau values as for a commensurate Bernal bilayer. This implies that the eightfold degeneracy of the zero energy mode is topologically protected despite rotational disorder as recently predicted. In addition, an anomaly appears. The densities at which these plateaus occur show a magnetic field dependent offset. It suggests the existence of a pool of localized states at low energy, which do not count towards the degeneracy of the lowest band Landau levels. These states originate from an inhomogeneous spatial variation of the interlayer coupling.

We report on infrared (IR) nanoscopy of 2D plasmon excitations of Dirac fermions in graphene. This is achieved by confining mid-IR radiation at the apex of a nanoscale tip: an approach yielding 2 orders of magnitude increase in the value of in-plane component of incident wavevector q compared to free space propagation. At these high wavevectors, the Dirac plasmon is found to dramatically enhance the near-field interaction with mid-IR surface phonons of SiO2 substrate. Our data augmented by detailed modeling establish graphene as a new medium supporting plasmonic effects that can be controlled by gate voltage.

@article{rodrigues_zigzag_2011,
title = {Zigzag graphene nanoribbon edge reconstruction with Stone-Wales defects},
author = {Rodrigues, J. N. B. and Goncalves, P. a. D. and Rodrigues, N. F. G. and Ribeiro, R. M. and Lopes dos Santos, J. M. B. and Peres, N. M. R.},
doi = {10.1103/PhysRevB.84.155435},
issn = {1098-0121},
year = {2011},
date = {2011-10-01},
journal = {Phys. Rev. B},
volume = {84},
number = {15},
pages = {155435},
abstract = {In this paper, we study zigzag graphene nanoribbons with edges reconstructed with Stone-Wales defects, by means of an empirical (first-neighbor) tight-binding method, with parameters determined by ab initio calculations of very narrow ribbons. We explore the characteristics of the electronic band structure with a focus on the nature of edge states. Edge reconstruction allows the appearance of a new type of edge states. They are dispersive, with nonzero amplitudes in both sublattices; furthermore, the amplitudes have two components that decrease with different decay lengths with the distance from the edge; at the Dirac points one of these lengths diverges, whereas the other remains finite, of the order of the lattice parameter. We trace this curious effect to the doubling of the unit cell along the edge, brought about by the edge reconstruction. In the presence of a magnetic field, the zero-energy Landau level is no longer degenerate with edge states as in the case of the pristine zigzag ribbon.},
note = {WOS:000296290000014},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

In this paper, we study zigzag graphene nanoribbons with edges reconstructed with Stone-Wales defects, by means of an empirical (first-neighbor) tight-binding method, with parameters determined by ab initio calculations of very narrow ribbons. We explore the characteristics of the electronic band structure with a focus on the nature of edge states. Edge reconstruction allows the appearance of a new type of edge states. They are dispersive, with nonzero amplitudes in both sublattices; furthermore, the amplitudes have two components that decrease with different decay lengths with the distance from the edge; at the Dirac points one of these lengths diverges, whereas the other remains finite, of the order of the lattice parameter. We trace this curious effect to the doubling of the unit cell along the edge, brought about by the edge reconstruction. In the presence of a magnetic field, the zero-energy Landau level is no longer degenerate with edge states as in the case of the pristine zigzag ribbon.

@article{rappoport_magnetic_2011,
title = {Magnetic exchange mechanism for electronic gap opening in graphene},
author = {Rappoport, T. G. and Godoy, M. and Uchoa, B. and Dos Santos, R. R. and Castro Neto, A. H.},
doi = {10.1209/0295-5075/96/27010},
issn = {0295-5075},
year = {2011},
date = {2011-10-01},
journal = {EPL},
volume = {96},
number = {2},
pages = {27010},
abstract = {We show within a local self-consistent mean-field treatment that a random distribution of magnetic adatoms can open a robust gap in the electronic spectrum of graphene. The electronic gap results from the localization of the charge carriers that arises from the interplay between the graphene sublattice structure and the exchange interaction between the adatoms. The size of the gap depends on the strength of the exchange interaction between carriers and localized spins and can be controlled by both temperature and external magnetic field. Furthermore, we show that an external magnetic field creates an imbalance of spin-up and spin-down carriers at the Fermi level, making doped graphene suitable for spin injection and other spintronic applications. Copyright (C) EPLA, 2011},
note = {WOS:000295974600033},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

We show within a local self-consistent mean-field treatment that a random distribution of magnetic adatoms can open a robust gap in the electronic spectrum of graphene. The electronic gap results from the localization of the charge carriers that arises from the interplay between the graphene sublattice structure and the exchange interaction between the adatoms. The size of the gap depends on the strength of the exchange interaction between carriers and localized spins and can be controlled by both temperature and external magnetic field. Furthermore, we show that an external magnetic field creates an imbalance of spin-up and spin-down carriers at the Fermi level, making doped graphene suitable for spin injection and other spintronic applications. Copyright (C) EPLA, 2011

@article{santos_electronic_2011,
title = {Electronic doping of graphene by deposited transition metal atoms},
author = {Santos, Jaime E. and Peres, Nuno M. R. and Lopes dos Santos, Joao M. B. and Castro Neto, Antonio H.},
doi = {10.1103/PhysRevB.84.085430},
issn = {1098-0121},
year = {2011},
date = {2011-08-01},
journal = {Phys. Rev. B},
volume = {84},
number = {8},
pages = {085430},
abstract = {We perform a phenomenological analysis of the problem of the electronic doping of a graphene sheet by deposited transition metal atoms, which aggregate in clusters. The sample is placed in a capacitor device such that the electronic doping of graphene can be varied by the application of a gate voltage and such that transport measurements can be performed via the application of a (much smaller) voltage along the graphene sample, as reported in the work of Pi et al. [Phys. Rev. B 80, 075406 (2009)]. The analysis allows us to explain the thermodynamic properties of the device, such as the level of doping of graphene and the ionization potential of the metal clusters, in terms of the chemical interaction between graphene and the clusters. We are also able, by modeling the metallic clusters as perfectly conducting spheres, to determine the scattering potential due to these clusters on the electronic carriers of graphene and hence the contribution of these clusters to the resistivity of the sample. The model presented is able to explain the measurements performed by Pi et al. on Pt-covered graphene samples at the lowest metallic coverages measured, and we also present a theoretical argument based on the above model that explains why significant deviations from such a theory are observed at higher levels of coverage.},
note = {WOS:000294325400014},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

We perform a phenomenological analysis of the problem of the electronic doping of a graphene sheet by deposited transition metal atoms, which aggregate in clusters. The sample is placed in a capacitor device such that the electronic doping of graphene can be varied by the application of a gate voltage and such that transport measurements can be performed via the application of a (much smaller) voltage along the graphene sample, as reported in the work of Pi et al. [Phys. Rev. B 80, 075406 (2009)]. The analysis allows us to explain the thermodynamic properties of the device, such as the level of doping of graphene and the ionization potential of the metal clusters, in terms of the chemical interaction between graphene and the clusters. We are also able, by modeling the metallic clusters as perfectly conducting spheres, to determine the scattering potential due to these clusters on the electronic carriers of graphene and hence the contribution of these clusters to the resistivity of the sample. The model presented is able to explain the measurements performed by Pi et al. on Pt-covered graphene samples at the lowest metallic coverages measured, and we also present a theoretical argument based on the above model that explains why significant deviations from such a theory are observed at higher levels of coverage.

Graphene is possibly one of the largest and fastest growing fields in condensed matter research. However, graphene is only one example in a large class of two-dimensional crystals with unusual properties. In this paper we briefly review the properties of graphene and look at the exciting possibilities that lie ahead.

We show that a twisted graphene bilayer can reveal unusual topological properties at low energies, as a consequence of a Dirac-point splitting. These features rely on a symmetry analysis of the electron hopping between the two layers of graphene and we derive a simplified effective low-energy Hamiltonian which captures the essential topological properties of a twisted graphene bilayer. The corresponding Landau levels peculiarly reveal a degenerate zero-energy mode which cannot be lifted by strong magnetic fields.

We show that Coulomb drag in ultra-clean graphene double layers can be used for controlling the on-and-off ratio for current flow by tuning the external gate voltage. Hence, although graphene remains semi-metallic, the double-layer graphene system can be tuned from conductive to a highly resistive state. We show that our results explain previous data of Coulomb drag in double-layer graphene samples in disordered SiO2 substrates. Copyright (C) EPLA, 2011

We demonstrate injection, transport, and detection of spins in spin valve arrays patterned in both copper based chemical vapor deposition (Cu-CVD) synthesized wafer scale single layer and bilayer graphene. We observe spin relaxation times comparable to those reported for exfoliated graphene samples demonstrating that chemical vapor deposition specific structural differences such as nanoripples do not limit spin transport in the present samples. Our observations make Cu-CVD graphene a promising material of choice for large scale spintronic applications.

@article{nayak_graphene_2011,
title = {Graphene for Controlled and Accelerated Osteogenic Differentiation of Human Mesenchymal Stem Cells},
author = {Nayak, Tapas R. and Andersen, Henrik and Makam, Venkata S. and Khaw, Clement and Bae, Sukang and Xu, Xiangfan and Ee, Pui-Lai R. and Ahn, Jong-Hyun and Hong, Byung Hee and Pastorin, Giorgia and Oezyilmaz, Barbaros},
doi = {10.1021/nn200500h},
issn = {1936-0851},
year = {2011},
date = {2011-06-01},
journal = {ACS Nano},
volume = {5},
number = {6},
pages = {4670--4678},
abstract = {Current tissue engineering approaches combine different scaffold materials with living cells to provide biological substitutes that can repair and eventually improve tissue functions. Both natural and synthetic materials have been fabricated for transplantation of stem cells and their specific differentiation Into muscles, bones, and cartilages. One of the key objectives for bone regeneration therapy to be successful Is to direct stem cells' proliferation and to accelerate their differentiation in a controlled manner through the use of growth factors and osteogenic inducers. Here we show that graphene provides a promising biocompatible scaffold that does not hamper the proliferation of human mesenchymal stem cells (hMSCs) and accelerates their specific differentiation into bone cells. The differentiation rate Is comparable to the one achieved with common growth factors, demonstrating graphene's potential for stem cell research.},
note = {WOS:000292055200049},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

Current tissue engineering approaches combine different scaffold materials with living cells to provide biological substitutes that can repair and eventually improve tissue functions. Both natural and synthetic materials have been fabricated for transplantation of stem cells and their specific differentiation Into muscles, bones, and cartilages. One of the key objectives for bone regeneration therapy to be successful Is to direct stem cells' proliferation and to accelerate their differentiation in a controlled manner through the use of growth factors and osteogenic inducers. Here we show that graphene provides a promising biocompatible scaffold that does not hamper the proliferation of human mesenchymal stem cells (hMSCs) and accelerates their specific differentiation into bone cells. The differentiation rate Is comparable to the one achieved with common growth factors, demonstrating graphene's potential for stem cell research.

@article{castro_neto_two-dimensional_2011,
title = {Two-Dimensional Crystals: Beyond Graphene},
author = {Castro Neto, A. H. and Novoselov, K.},
doi = {10.1166/mex.2011.1002},
issn = {2158-5849},
year = {2011},
date = {2011-03-01},
journal = {Mater. Express},
volume = {1},
number = {1},
pages = {10--17},
abstract = {Human progress and development has always been marked by breakthroughs in the control of materials. Since pre-historic times, through the stone, bronze, and iron ages, humans have exploited their environment for materials that can be either used directly or can be modified for their benefit, to make their life more comfortable, productive, or to give them military advantage. One age replaces another when the material that is the basis for its sustainability runs its course and is replaced by another material which presents more qualities. Multi-tasking, speed, versatility, and flexibility are at the heart of modern technology. In recent years a new class of materials that can fulfill these needs have emerged: two-dimensional (2D) crystals. Graphene is probably the most famous example, but there are numerous other examples with amazing electronic and structural properties. In this paper we look into the possible routes for exploration of this new field that presents new venues in basic science as well as in applications.},
note = {WOS:000311866900002},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

Human progress and development has always been marked by breakthroughs in the control of materials. Since pre-historic times, through the stone, bronze, and iron ages, humans have exploited their environment for materials that can be either used directly or can be modified for their benefit, to make their life more comfortable, productive, or to give them military advantage. One age replaces another when the material that is the basis for its sustainability runs its course and is replaced by another material which presents more qualities. Multi-tasking, speed, versatility, and flexibility are at the heart of modern technology. In recent years a new class of materials that can fulfill these needs have emerged: two-dimensional (2D) crystals. Graphene is probably the most famous example, but there are numerous other examples with amazing electronic and structural properties. In this paper we look into the possible routes for exploration of this new field that presents new venues in basic science as well as in applications.

The optical conductivity of graphene strained uniaxially is studied within the Kubo-Greenwood formalism. Focusing on inter-band absorption, we analyze and quantify the breakdown of universal transparency in the visible region of the spectrum, and analytically characterize the transparency as a function of strain and polarization. Measuring transmittance as a function of incident polarization directly reflects the magnitude and direction of strain. Moreover, direction-dependent selection rules permit the identification of the lattice orientation by monitoring the van Hove transitions. These photoelastic effects in graphene can be explored towards atomically thin, broadband optical elements. Copyright (C) EPLA, 2010